stm32mp1: update platform files

Move print_reset_reason function to bl2_plat_setup.c
Put __unused attribute for unused bl2_el3_early_platform_setup args.
Rename dt_dev_info to dt_uart_info.
Put MMU configuration earlier.
Remove unused macros.
Use U() or ULL() macros where needed.
Use device tree to configure GIC.
Use GIC helper function.

Change-Id: I34620c421cc6967a668bca318f7689fd74fa78a6
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>